Squirrel-cage rotor for induction motors



June 10, 1930. G. 'r. SMITH 1,762,564

' SQUIRREL cm: ROTOR FOR mnucnon MOTORS Filed April 6, 1929 2 sheds-sheet 1 INVENTOR acid! BY:

ATTORNEY June 10, 1930- G. 1'. SMITH SQUIRREL CAGE ROTOR FOR INDUCTION MOTORS Filed April 6, 1929 2 Sheets-Sheet 2 INVENTOR ATTORNEY Patented June 10, 1930 PATENT OFFICE GEORGE T. SMITH, OF INDIANAPOLIS, INDIANA SQUIRREL-CAGE ROTOR FOB INDUCTION MOTORS Application filed April 6,

The present invention relates to improvements in squirrel cage rotors for induction motors.

It is well known that in such motors, un-

less provision is made to overcome the difliculty, a very large starting current is necessary and such current does not decrease materially until the motor has developed considerable speed. Devices have been made to take care of this difliculty but they themselves are not without objection. The chief form of device for this purpose is a switching device which provides for a high resistance in the rotor winding at starting and up to a 5 predetermined speed with means for cutting out the resistance when the motor caches such speed. This, however, does not provide the most desirable conditions.

Among the objects of the present invention is to accomplish the same change from high resistance to low with increase of speed and to make such change gradually and as nearly as possible in inverse proportion to the speed.

Another object is such a construction as will automatically nrovide for proper current limiting on starting and therefore not require the use of an auto-transformer type or resistance type of starter.

Other objects will readily occur to those skilled in the art upon reference to the following description and the accompanying drawings in which Fig. 1 is a longitudinal section through one 35 form of motor embodying the present invention.

Fig. 2 is an end view of the same with part in section on line 22 of Fig. 1.

Fig. 3 is a partial section on line 3-3 of Fig. 4 is a longitudinal section illustrating a somewhat different embodiment of the invention.

Fig. 5 is an elevation of the expanding rings of Fig. 4.

Fig. 6 illustrates in section another embodiment.

Fig. 7 is a partial view showing one form 50 of high resistance ring.

1929. Serial No. 352,971.

Fig. 8 is a longitudinal sectional view showing a further modification.

In the drawings the stator of the motor is indicated by the portions 20 of the pole pieces, no further stator construction being shown as the invention relates solely to the rotor.

The rotor is shown as comprising the usual core 21 mounted upon a spider 22 carried on shaft 23 and held between the end pieces 24 and 25, the latter being extended as at 26 to provide means for mounting certain parts to be described.

Extending lengthwise of the core 21 are the squirrel cage winding elements which include the bars 30 and at one end these may be joined by brazing or otherwise securing thereto. in conductino relation, a low resistance ring 31. At the other end of the rotor the bars 30 are connected by means of a high resistance ring 32 likewise brazed, soldered, or otherwise secured thereto, with the bars extending a short distance beyond.

The overhanging ends of the bars 30 lie between and are spaced a short distance from a plurality of rings 35, Within the circle of the bar ends, and a flanged ring 36 mounted on the end of extension 26 of end piece 25 andoverlying the circle of bar ends.

The rings 35 are expansible and made of more or less resilient material of low resistso ance while the ring 36 is made quite rigid and also of low resistance.

When, therefore, the rotor is still or turning at low speed, only the high resistance ring 32 connects the bars at this end. But when the speed of rotation has increased to the point where the rings 35 expand, due to centrifugal action, they contact with the bar ends and the resistance to current flow be tween the bars is thereby reduced. The light pressure between the rings 35 and the bars 30 is increased as the speed increases and, as the pressure increases, the resistance at the points of contact decreases.

As the speed incre :868 the rings 35 press the bars outwardly and against the ring 36 to thereb further decrease the resistance, while the ring 36 also acts as a support for the bars 30 to sustain them against the outward thrust of the rings 35.

Fig. 4 shows a slightly modified form of the invention wherein the rings are substituted by a single one or a plurality of helical rings 35, an elevation of which is shown in Fig. 5.

Fig. 6 shows a further modification in which the flanged ring 86 is omitted and the outward thrust of rings 35 taken by a flat steel hoop 36 surrounding the extended ends or bars 30 and insulated therefrom as at 37.

Fig. 8 shows a further modification in that there are two sets of longitudinal bars 30* and 30". f these, the former 8O are high resistance bars and are connected at both ends by high resistance rings 32. This winding operates chiefly from standstill and during the lower speeds before the centrifugal force has caused reduced resistance in the winding indicated by the bars 30 The latter are connected at one end by a low resistance ring 31 and at the other end provided with flexible portions 30, weighted as at 85, so that these ends are thrown out against the low resistance flanged ring 36 when the rotor speed is suflicient.

Fig. '7 shows in elevation the preferred form of high resistance ring as used at 82 and 32 in the other figures.

By the use of the present invention, and uitably proportioning the ring 35 the change in the wiring conductivity may be gradually and uniformly changed from the desired high resistance at standstill and low speeds, to low resistance at the desired operatin speed and hereby obtain more current consumption.

N ow having described the invention and the preferred forms of embodiment thereof, it is to be understood that the said invention is to be limited not to the specific details herein described and illustrated, but only by the scope of the claim which follows:

In a motor, a rotor having a plurality of separated parallel bars attached thereto and having flexible extensions thereon, a resilient centrifugally expanding low resistance ring attached to said rotor and underlying said we tensions, a high resistance ring surrounded by said extensions, and a low resistance ring surrounding said extensions, the high resistance ring electrically interconnecting said bars when the rotor is at rest or revolving at a low speed, the low resistance resilient ring expandin when the rotor revolves at a high speed to nrst cause said bars to move away from said high resistance ring, and to connect the bars to each other thru the resilient ring, and then upon rotation at a still higher speed, to cause said bars to be connected to each other thru said low resistance ring.

GEORGE T. SMITH. 

